Remove the EmbeddedVTTStylesheetsEnabled runtime flag

The CL removes the EmbeddedVTTStylesheetsEnabled runtime flag as the
feature has been stable since M78.

Bug: 1056392
Change-Id: I935bf5f099753f5e33b312b5aca1a513a7c29152
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076408Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#745098}

third_party/blink/renderer/core/html/track/vtt/vtt_parser.cc

@@ -285,10 +285,8 @@ VTTParser::ParseState VTTParser::CollectWebVTTBlock(const String& line) {
     if (line.StartsWith("STYLE") && StringView(line, kStyleIdentifierLength)
                                         .IsAllSpecialCharacters<IsASpace>()) {
       contains_style_block_ = true;
-      if (RuntimeEnabledFeatures::EmbeddedVTTStylesheetsEnabled()) {
-        current_content_.Clear();
-        return kStyle;
-      }
+      current_content_.Clear();
+      return kStyle;
     }
   }
 

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -618,10 +618,6 @@
       // http://crbug.com/707656 content editable in LayoutNG.
       name: "EditingNG",
     },
-    {
-      name: "EmbeddedVTTStylesheets",
-      status: "experimental",
-    },
     {
       name: "EncryptedMediaEncryptionSchemeQuery",
       status: "stable",